Photocurable compositions are cured by exposure to radiation, usually ultraviolet radiation, and include for example, lacquers which may be applied to wood, metal or similar substrates by suitable techniques such as roll coating or curtain coating. They may also be formulated as inks, for example to be applied by techniques such as letterpress, offset lithography, rotogravure printing, silk screen printing, flexographic or ink jet printing. Printing, depending on the particular printing technique, is applicable to a wide range of substrates which include paper, board, glass, plastics materials or metals.
Such compositions will contain the monomer or oligomer to be polymerised, together with a photoinitiator, whose function is to absorb the radiation, and form an excited state which can then initiate polymerisation. In addition, there may be a sensitiser, whose function is to enhance and/or broaden the absorption spectrum of the initiator. Where, as is common, the composition is to be used in liquid form, there may also be a solvent/viscosity modifier, which is preferably also polymerisable. However, it is normally preferred to avoid any such additive, if possible, as it may modify the properties of the final polymerised coating in unpredictable or undesirable ways.
Michler's ketone is the most well-known sensitiser for radiation curing. Although it can function as a photoinitiator in its own right, Michler's ketone is not particularly efficient as it has a significant charge transfer character to its lowest lying triplet state. This makes hydrogen abstraction from donor molecules unfavourable due to the high electron density on the carbonyl oxygen. However, a combination of benzophenone and Michler's ketone acts as a synergistic combination due to the formation of an excited state complex which can be populated by excitation of either molecule.
We have now surprisingly found a series of piperazine compounds which can be used with Type II photoinitiators to provide extremely efficient radiation cure.
Certain compounds similar to those of the present invention have been disclosed in U.S. Pat. No. 6,022,906. However, these are proposed for use as photoinitiators, whereas we have shown that the compounds of the present invention are relatively ineffective as photoinitiators whilst having powerful sensitising activity.